Rolling take-off 737
 

Reading the 737 FCTM it states I was wondering if anyone has any knowledge of what a ‘rolling takeoff’ actually is in this context? For example, if you have a 90 degree line-up, are they saying that the ‘square turn’ to align with the runway followed by being stationary when pressing TOGA provides negligible advantage to simply rolling out onto the runway at 8kts with 40% set and pressing TOGA 50 or 60 m further down the runway?

I suspect that entering the runway via a RET could potentially offer quite a bit of advantage.

I’m not familiar with the abbreviation David. What’s a RET?

RET = Rapid Exit Taxiway. ie. 15 to 30 degrees off the heading of the rumway.
If you line up via one of these then you can have 20 knots or more before you start the take off roll.

Theoretical considerations:

20 knots =37km/h=10 m/s
Assuming the 737 accelerates at 0.2g, that takes approximately 5 seconds / 25 meters / 80 feet, which is the runway distance you could save.

"rolling out onto the runway at 8kts with 40% set and pressing TOGA 50 or 60 m further down the runway" would be worse?
8 knots is about 2 seconds of acceleration, so 4 meters of runway?

I've not considered engine spoolup here, but in that regard, putting the brake on and going to 70% before commencing the roll is probably beating any rolling start.

ahhh ok thanks. I’m not really interested in that as much as the difference between a 90 degree turn onto the centerline, brining the thrust to 40% while stationary v’s rolling onto the runway at 8-12 kts with engines spooled up at 40% and then pressing TOGA once aligned with the centerline. Obviously if you roll out at taxi speed you are chewing up runway distance but I think what Boeing is saying is that the kinetic energy you have when you press TOGA using that method is similar ( negligible difference) to the energy you would have at the same spot if you had done a 90 degree line up. Where are the physics geeks?

They're not wasting their time because as Boeing says, it makes no difference.

The consideration is to think upon the point at which the two techniques would put you at the same speed with TO thrust set. If you are heavy, then the rolling start with a prompt spool up and acceleration is going to put you ahead. If you are light, then you could loss quite some difference in length unless the rolling acceleration is made to be very prompt. So far as spool up on a normal intersection, going to 40% with 90° to go is counterproductive - uncomfortable, tyre scrub wear, etc. What you do, in practice, is taxy at a normal speed and, while the aircraft is turning towards the runway heading, spool up and, by the time you are near aligned, you are accelerating fairly quickly.

The critical consideration is the promptness with which you get the thing going. Should you spool up slowly, take all day to accelerate gently and slowly to TO thrust conditions, then you could put yourself in an awkward position on a short runway. Providing you are prompt and decisive, it works real fine.

putting the brake on and going to 70% before commencing the roll is probably beating any rolling start.

.. and great for FOD unless you are on a nice clean runway surface.....

And at many airfirelds the runways are so busy that they would not really appreciate your stopping, then setting power from a standing start. Just a thought.

I'm not sure how useful this is, but this is the physics you asked for (I hope):

Kinetic energy grows with the square of the speed.
Therefore, if you have 10% of your final speed, you have only 1% of your final kinetic energy. Neglible.
(Regardless of weight.)

Rotational energy of the turbines is proportional to the square of the rpm, so 30% vs 40% N1 would be 9% vs 16% of the rotational energy of the turbines at 100%.

Clearly on a short runway, where takeoff distance is important, a standing start probably saves some tens of metres. But on a long runway, TODA may exceed a 737’s TODR by upwards of 1,000m: in that case, is a standing start still preferable if you consider other aspects such as fuel burn, stress on the braking system, and (as john_tullamarine says) FOD?

I agree, the kinetic (physical) difference between rolling and standing start is neglible, the operational differences matter more.

(I had the recent de-icing question in my mind, which involved a 70%/30s engine runup on the runway, and limited my answer to the physical aspect only ("theoretical considerations", as opposed to practical advice). I'm sorry if that wasn't clear.)

The answer is on the next page in your FCTM. It is all about the initial spool-up of the engines to 40%N1. In short:

Rolling:
No use of brakes to achieve 40%N1, when aligned and on centerline, release nose wheel steering, and set takeoff thrust. This can be done either during the alignment from the taxiway (has nothing to do with high-speed exit taxiways by the way, the goal is not to accelerate on the taxiway, Boeing is clear it is to be done with "normal taxi speeds"), or also when already positioned on the runway.

Standing:
Use of brakes to achieve 40%N1, when engines are stable ensure the nose wheel steering is released, then release brakes and promptly set takeoff thrust.

Couldn’t agree more, a careful line up leaving the least runway behind you while keeping your momentum going, min spool in the turn then selecting take off power as soon as you’re lined up gives you the maximum advantage


Less inertia to overcome with some forward speed and take off thrust set as close as possible to the beginning of the runway

If I’ve done intersection data (expected or no difference in de-rate) and then get full length it makes it a lot easier as I can roll as far as next intersection before setting thrust. As someone says depends on runway length, perf in hand, minimum runway occupancy requirements etc. But on a heavy you can’t set 55% on the line up - need to wait until lined up or it’s pretty messy.

We have switched to using OPT for performance and you can display a nice little diagram showing AEO, OEI and V1 stop distances along with TORA, TODA and ASDA so you can immediately see your margins and which technique would be appropriate in the circumstances. A rolling takeoff is SOP for us but if I’m going out of a balanced field with a 90deg turn on then I’ll look to minimise the runway used in the line up.

Good point. The thing I don’t like is when people line up and then slowly trundle down the runway towards the next intersection while waiting for takeoff clearance. This uses up runway unnecessarily. Bring it to a stop and then when cleared release the brakes and do a rolling takeoff (unless an icing runup is required). A rolling takeoff reduces the chances of an engine surge or stall as it ensures the airflow is going the correct way through the engine as it spools up.

Caution wet paint and area of black tire deposits…
 

Did a rolling start in Seoul years ago after being asked by tower if I would accept an immediate takeoff due to inbound 747.
looked easy enough during the turn, I brought the engines up, and as luck would have it, the left engine (outside of the turn) spun up rather sooner than the right engine and we started sliding toward the right side of the runway and I could find no traction at all. Went right to the edge of the runway before getting any turning capability back to the center.

with my not knowing whether I had hit a rwy edge light and not knowing if I would have felt it, I requested to abort the takeoff and return to the gate.

as it turned out, our main gear had perfectly went around a rwy edge light just up to the edge of the pavement and the pathway was visible for all to see on the right side rwy at Kimpo airport. 🤣